What hours and days are you in each port? 8am-5pm? or differs. Very important as locations hours vary.
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Well, here's what I'd do, based on taking a Hawaiian cruise several years ago, plus several "regular" trips to the Islands:

Hilo - drive up to Volcano National Park. Kona side is great, but you may not have enought time to hit the volcano and Kona.

Maui - drive out to Lahaina/Kaanapali area and enjoy some great beaches

Kaui - last time we were there on a cruise, we got in too late to take one of the excursions up to the Na Pali coast, so we ended up taking a helicopter ride around the island to get a good look/feel for it.

Oahu - drive around the island, head east to snorkel in Hanauma Bay, then continue on up the North Side, and back to Waikiki. Since you have a car, you should be able to pop out to La Mariana for drinks

Since my trip is now in the foreseeable future (8 months) I've started actively planning. My itinerary is a little bit different. I'll be flying into Oahu, spending 2+ days there before getting on the ship. We port at Maui (Kahlua) from Sunday 8am to Monday 6pm then Hilo on Tuesday from 8am to 6pm, Kona Wed. 7am - 5:30pm, and Kauai from 8am Thursday to 2pm Friday before sailing back to Oahu and flying home late Saturday. We'll be renting a car at most, if not all, ports depending on what we plan. We'll definitely have a car on Oahu.

I've written down all the comments so far and am compiling a giant list. Please keep the suggestions coming and Mahalo for all who have contributed thus far!

Lori, by "8 months from now" do you mean September? My wife, adult daughters and I will be on Oahu the last 2 weeks of September. If you're there at the same time, maybe we can all meet for Mai Tais!
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